The present invention relates to a process for the preparation of N-alkylanilines, in which anilines which are not substituted on the N atom are reacted with N,N-dialkylanilines in the presence of zeolites at elevated temperature.
N-Monoalkylated anilines are important industrial intermediates for the preparation of dyestuffs, stabilizers, urethanes or ureas. They are prepared by alkylation of anilines with alcohols or the corresponding dialkyl ethers in the presence of acid catalysts, for example in the presence of phosphorous oxychloride, under pressure. However, not the pure monoalkylated anilines were produced by this way, but always also N,N-dialkylanilines to a large extent (Ullmanns Encyclopadie der technisechen Chemie (Ullmann's Encyclopaedia of Industrial Chemistry), 4th edition, Vol. 7 (1974), p. 572 ff.).
The ratio of monoalkylated to dialkylated anilines, such as is obtained in these preparation processes, thus, however, not always conforms to the particular demand. In general, the monoalkylated anilines are the more desirable products, which at the same time are more difficult to obtain. Moreover, N,N-dialkylanilines can be obtained in good yields by special processes. It is therefore desirable to render the ratio of monoalkylated to dialkylated anilines more flexible, that is, to have an efficient process for converting N,N-dialkylanilines into N-monoalkylanilines available. According to Acta Chim. Hung. 20 (1959), 321, transalkylation between N,N-diethylaniline and aniline can be achieved by passing the two reactants at elevated temperature in the gas phase over Al.sub.2 O.sub.3. However, this transalkylation proceeds very slowly. At a space velocity of 0.25 ml/ml of catalyst/h at 250.degree. C., only 0.16 mol % are converted. If the temperature is increased to accelerate the reaction, the conversion of N,N-diethylaniline is increased, but more and more ring-alkylated products appear and aniline is practically not alkylated on the N atom (loc. cit., p. 322, Table II). If at 280.degree. C. the space velocity is only slightly increased, the conversion drops noticeably and yet non-negligible amounts of ring-alkylated products are formed (loc. cit., p. 323, Table III).
JP No. 60/045,552-A proposes to carry out the transalkylation between N,N-dimethylaniline and aniline in the presence of considerable amounts of concentrated sulphuric acid and a large excess of aniline at 180.degree. to 300.degree. C. in an autoclave. This reaction requires several hours; the H.sub.2 SO.sub.4 catalyst has to be neutralized (disposed of) with alkali, and gets lost; finally the conversion is still incomplete despite a large amount of catalyst and excess of aniline. Furthermore, a reaction under pressure is in general associated with particular technical expenditure and, in the present case, also with corrosion risks.